VCE Environmental Science (Year 12) · Unit 3 — Ecosystems and Processes and Human Impacts · 16m read

Threats to Biodiversity and Extinction Risk

Australia has one of the worst mammal extinction records in the world. Understanding why species decline — and why small populations are especially fragile — is essential before you can judge how to save them.

Learning objective: Explain major threats to biodiversity and the factors that increase a species’ risk of extinction.

By the end of this lesson you can:

• I can explain the major threats to biodiversity.
• I can explain why small, isolated populations are vulnerable.
• I can interpret conservation status categories.

Threats to biodiversity are hammers on a Jenga tower. Habitat loss, invasive species, pollution, population and overharvesting (HIPPO) each knock blocks out.

Why species decline

MAJOR THREATS (often remembered as HIPPO):
• HABITAT loss, fragmentation and degradation.
• INVASIVE species (foxes, cats, rabbits, weeds, pathogens such as chytrid fungus).
• POLLUTION of air, water and soil.
• POPULATION growth and resource consumption by humans.
• OVER-EXPLOITATION (hunting, overfishing, harvesting).
• Plus CLIMATE CHANGE, which shifts habitats and increases extreme events.

WHY SMALL POPULATIONS ARE VULNERABLE:
• Loss of GENETIC DIVERSITY through genetic drift and inbreeding.
• Inbreeding depression reduces fitness.
• Chance events (fire, disease) can wipe out a small population.
• Fragmentation isolates populations and blocks gene flow.

CONSERVATION STATUS:
• IUCN Red List categories include Least Concern, Near Threatened, Vulnerable, Endangered, Critically Endangered, Extinct in the Wild and Extinct.
• In Australia, species are listed nationally under the EPBC Act.

SPECIES TRAITS THAT INCREASE RISK:
• Small range, specialised diet or habitat, slow reproduction, large body size.

Quick reference

| Threat (HIPPO) | Example |
|---|---|
| Habitat loss | Land clearing |
| Invasive species | Foxes, cats, cane toads |
| Pollution | Plastics, runoff |
| Population (human) | Resource demand |
| Overharvesting | Overfishing |
| Climate change | Range shifts, bleaching |

Key ideas at a glance

• Multiple interacting threats: Cumulative.
• IUCN categories: LC → EX.
• Small populations: Genetic bottlenecks.
• Endemic species: High risk.

Worked example (Medium)

Explain why habitat fragmentation can threaten a population even when the total habitat area is unchanged.

1. Isolation. Fragments split one population into several small, isolated ones.
2. Genetics. Gene flow stops; small groups lose genetic diversity and suffer inbreeding.
3. Edge effects. More edge exposes animals to predators, weeds and changed microclimates.

Answer: Fragmentation creates small isolated populations with reduced gene flow and stronger edge effects, raising extinction risk.

Common mistakes

• Naming a threat without a mechanism Marks are for explaining HOW it causes decline. Fix: Link the threat to survival or reproduction.
• Ignoring genetics Genetic diversity is central to the study design. Fix: Explain drift, inbreeding and loss of adaptive potential.

Australia's extinction record

Australia has one of the world's worst records of mammal extinctions since European colonisation, largely due to feral cats and foxes and habitat loss.

Predator-proof fenced sanctuaries are now helping species like bilbies recover.

Practice questions

1. Which trait most increases a species’ extinction risk?

   1. Wide geographic range
   2. Generalist diet
   3. Slow reproductive rate
   4. Large population

   (1 marks)

Summary

• Threats: habitat loss, invasive species, pollution, over-exploitation, climate change.
• Small isolated populations lose genetic diversity.
• Status categories guide conservation priorities.